Jet engine igniters are used, in a manner similar to automobile spark plugs, to ignite an air-fuel mixture in the combustion chambers of gas turbine engines. Igniters typically comprise two concentric electrodes separated by an insulator, for example, aluminum oxide. A high voltage is applied to the central electrode to initiate an electric discharge in the air-fuel mixture. Current in the electric discharge then rises to deliver sufficient energy to initiate ignition of the mixture.
Jet aircraft engine igniters are utilized during engine startup and are, additionally, operated as a precaution against flame-out during take off, landing, and poor weather conditions. Typically, an igniter is operated approximately ten percent of engine running time.
Igniters for engines in heavy jet aircraft typically operate under particularly severe conditions. For example, in the General Electric Company CF6-50 engine, which powers the McDonald-Douglas DC 10 aircraft, a power supply (the exciter) delivers brief, high voltage pulses to the igniter with a pulse energy in the range of from 1 to 2 joules at a repetition rate of 2 pulses per second. The igniter must operate over a pressure range from approximately 5 psia to over 200 psia at shell temperatures which range to approximately 2000.degree. F. Frequently, igniters become covered with liquid jet fuel under cold starting conditions. The lifetime of an igniter is, therefore, limited to approximately 100 hours of exciter operation.
The insulator in prior art igniters has frequently been shunted with a body of semiconductor material, for example, a thin film on the insulator surface. Such "shunted igniters" have been found to fire at substantially lower voltages than unshunted igniters and thus tend to reduce the weight and cost of associated exciter circuits. The high power required for reliable ignition in heavy jet engines has, however, been found to cause a rapid erosion of the semiconducting film, a condition which leads to unreliable ignition.